1. Field of the Invention
This invention relates to a diagnostic X-ray apparatus using digital X-ray subtraction imaging techniques including digitization of the video signals from an image intensifier--TV chain, digital subtraction, contour enhancement and reconversion to analog signals for viewing in real time on a monitor.
2. Description of the Prior Art
The digital X-ray subtraction imaging technique using the imaging intensifier--TV chain was developed and firstly described by Mistretta et al. For example, U.S. Pat. Nos. 4,204,225 and 4,204,226 respectively issued to Mistretta and Mistretta et al disclose realtime digital X-ray subtraction imaging methods and apparata which employ the so-called Digital Radiography or Digital fluoroscopy technology. These digital X-ray subtraction imaging apparata consist of an image intensifier, a TV camera whose output is converted into digital format, and an image processor incorporating two digital memories.
As disclosed in U.S. Pat. No. 4,204,225, in carrying out the mask mode, a first memory system is preferably employed to integrate and store the digital mask video signals by digitization of the video signals from an image intensifier--TV chain during an initial mask time interval corresponding to a relatively large number of television fields, and a second memory system is employed to perform the functions of integrating the ongoing digital video signals and providing an output of the previously integrated video signals, from which the mask video signals from the first memory system are subtracted. The resulting digital difference video signals are supplied to a D/A convertor which provides corresponding analog difference video signals for display by the television display device, and are alternatively connected to a multiformat camera for making a selected number of radiographic exposures on a single film.
In the time interval difference mode disclosed in U.S. Pat. No. 4,204,226, a continuous series of difference images are produced by integrating digital video signals over a series of successive time intervals, performing a series of subtractions between each set of video signals integrated by each memory system and the preceding set to produce a series of successive digital difference video signals, and converting such digital difference video signals into visible television difference images representing changes in the X-ray image between the successive time intervals.
One advantage of a digital radiography apparatus is its capability to perform angiography by means of intravenous injection thereby supplanting intraarterial catheter techniques considered too risky to be used. Another advantage of the digital radiography apparatus is its capability to provide improved low contrast detectability, namely, by amplifying subtle amounts of contrast media in arteries better than possible using film methods. A further advantage is that the digital radiography apparatus employing a pulsed X-ray can prevent significant loss of spatial resolution due to physiological motion because the short radiation pulse for each image results in less motion unsharpness during fluoroscopy of moving objects.
However, in these digital radiography apparata the use of subtraction of different TV frames read out in an interlaced manner by a TV camera is affected by characteristics of the TV camera. In general, the TV camera using a pick up tube, such as the vidicon, has persistance characteristics shown in FIGS. 1a and 1b. The curve of FIG. 1a illustrates the relationship between rise time (after a step charge in target illumination) and relative signal output, and the curve of FIG. 1b illustrates the relationship beteeen decay time (after a step charge in target illumination) and relative signal output.
Therefore, this lag causes a difference in signal level between signals read out during a pulsed X-ray exposure and signals read out during the following pulsed X-ray exposure. For each television frame, it is necessary to maintain consistent image signal level and image information so as to remove or suppress effects by uninteresting soft tissue, bone, or air filled structures that overly and underly the structures of interest in the subtraction operation.